The present invention relates to the use of particles to prevent skidding and in particular to the prevention of skidding on pavement marking sheets.
Pavement marking sheets or tapes are well known to convey information to drivers of motor vehicles and pedestrians. The marking sheets typically have a highly visible color and often employ retroreflective particles for nighttime use.
Representative of pavement-marking sheet materials are U.S. Pat. No. 4,117,192 to Jorgensen (also disclosing irregular skid-resistant materials); U.S. Pat. No. 4,248,932 to Tung (disclosing skid-resisting irregular or angular inorganic particles, and optionally retroreflective spheres, scattered over the surface as being more skid-resistant than dense packed particles. The disclosed sheet material has a skid-resistance of at least 55 BPN); and U.S. Pat. No. 4,758,469 to Lange (disclosing an embossed retroreflecting pattern on a marking tape with optional embedded skid-resisting particles.
Anti-skid properties are desirable on many surfaces and are particularly desirable in pavement marking tapes in order to reduce slipping by pedestrians, bicycles, and vehicles upon the tapes. Several European governments have specified desirable levels of skid-resistance properties for pavement marking sheet materials. The specifications call for a minimum level of initial skid-resistance and retention of a significant level of skid-resistance "in use." When installed upon a pavement surface, (ie. "in use"), the marking tapes are subjected to an environment which is quite severe.
In the pavement marking industry, skid-prevention properties are typically imparted by embedding hard, crystalline particles with sharp points on an upper surface of the pavement marking tape. Typical examples of such conventionally employed particles include corundum (aluminum oxide) and quartz (sand, silicon dioxide, or micronized quartz). These particles are capable of achieving relatively good initial skid-resistance, however, the initial skid-resistance begins to decrease rapidly once the marking tape is exposed to traffic. Specifically, the impact of tires, the abrasion of loose dirt and sand, and the corrosion of salt and water contribute to loss of skid-resistance. The loss of skid-resistance is typically caused by some combination of at least one of two mechanisms. First, the crystalline materials have a tendency to fracture along crystalline planes. Second, the particles may become loosened from the embedding matrix.
One type of pavement marking tape, which employs a tough, durable top layer which resists wear or ablating and includes partially embedded crystalline particles to impart skid-resistance is disclosed in U.S. Pat. No. 4,020,211 to Ludwig Eigenmann. The Eigenmann product is a pavement marker with a plurality of particles bonded to a layer and projecting outwardly. The particles have a minimum hardness of about 6 on the Mohs Hardness Scale and include a pointed end portion projecting outwardly for imparting good anti skid properties. However, poor retention of acceptable skid-resistance in use has been observed.
An alternative product attempts to counter the above mechanisms by providing an ablative pavement marking sheet which has crystalline particles distributed throughout the thickness of the marking sheet, thereby continuously exposing fresh skid-resistant particles as the marking sheet wears. One drawback to the ablative marking tapes is maintaining an acceptable and informative color "in use," since ablative materials are typically soft and tend to become embedded with dirt. (See U.S. Pat. No. 4,490,432).
A type of particle, previously unknown to the pavement marking industry is disclosed in U.S. Pat. No. 4,680,230. The ceramic particles are taught to be useful as proppants in oil well hydraulic fracturing to improve pumping production. The particles comprise a vitreous matrix phase containing a crystalline alumina phase. The particles are characterized by a Krumbein roundness of at least 0.8 (1.0 representing a perfect sphere) and are highly chemically stable.
Canadian Patent No. 1002803 discloses an iron paving slab with recesses into which a binder and pulverized material, such as a ceramic is coated.
Japanese Patent No. 60130660 discloses a melt application type non-slip material which is filled with ceramic aggregate of baked silica, alumina, or clay and pigments.
Swiss Patent No. 541674 another Eigenmann patent discloses the use of globules of a binding material containing corundum (Aluminum Oxide).
West German Patent No. 2927362 discloses a material for abrasion resistant surfaces comprising glass powder embedded in a polymer matrix.